Women recovered from breast cancer are at increased risk for colorectal neoplasia. The reasons may be genetic, dietary or endogenous hormonal risk factors. Measurements of rectal epithelial proliferation are a useful biomarker of risk for large bowel cancer. This was studied in 12 women after (mean 7.8 years) cured breast cancer, who had a mean % labelling index of 7.5 +/- 3.5 (S.D.) as compared to 5.8 +/- 1.8 (S.D.) in a disease-free comparison group of 25 women. In addition, analysis of labelled crypt compartments demonstrated a significantly higher proportion in the study group with thymidine uptake, mainly in the mid crypt zone, and an extension of crypt cell DNA synthesis towards the surface epithelium. Using proliferative activity as a biomarker of risk in a larger study group, we may learn more about common etiological factors for both malignancies and also identify a higher-risk subgroup for long-term follow-up and possible therapeutic intervention.